Low temperature fractionation of gaseous mixtures with preliminary and split stream heat exchange



July 7, .1970 R. BECKER 3,518,839

LOW TEMPERATURE FRACTIONATION OF GASEOUS MIXTURES WITH PRELIMINARY AND SPLIT STREAM HEAT EXCHANGE Filed March 31, 1967 2 Sheets-Sheet l MIXING 52 64 CHAMBER 62 ea 59 Z PURSE --ADSORBER OUTLET 56 K] INVENTOR H G RUDOLF BECKER 1 ATTORNEY United States Patent 3,518,839 LOW TEMPERATURE FRACTIONATION 0F GAS- EOUS MIXTURES WITH PRELIMINARY AND SPLIT STREAM HEAT EXCHANGE Rudolf Becker, Munich, Germany, assignor to Linde Aktiengesellschaft, Wiesbaden, Germany Filed Mar. 31, 1967, Ser. No. 627,434 Claims priority, applicatiog 2Germany, Mar. 31, 1966,

5 50 Int. Cl. F25j 5/00 US. CI. 62-13 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates generally to a process and apparatus for cleaning and fractionating gaseous mixtures at low temperatures, and in particular to the separation of air into enriched streams of oxygen and nitrogen.

In the prior art, gaseous mixtures have been cleaned by cooling same in heat exchangers which cool and clean the mixture and congeal the impurities contained therein. In one such prior art process, the gaseous mixture is first cooled in a set of interchangeable, reversible or regenerative heat exchangers. To achieve very small temperature differences at the outlet, and to facilitate the vaporization of condensed impurities in the next cycle, a minor portion of the gaseous mixture in a partially cleaned, partially cooled state is withdrawn at a point intermediate the inlet and outlet of the heat exchangers. This partially cleaned withdrawn portion is then passed into a sheet and tube heat exchanger, or the like, in indirect countercurrent heat exchange relationship with the fully cooled and cleaned gas whereby the remaining impurities in the partially cleaned gas are congealed. The primary problem involved in the aforedescribed process is that deposits build up in the countercurrent heat exchanger, so it must be periodically shut down for cleaning. Consequently, it has been standard practice, heretofore, to provide at least two countercurrent heat exchangers in parallel, so that, as the cleaning of one becomes necessary, the other may be switched into line to take over the function of the former. The requirement for two countercurrent heat exchangers of the prior art is a particular dis advantage in that such a unit is very expensive.

SUMMARY OF THE INVENTION This invention provides a process and apparatus whereby a single countercurrent (crosscurrent) heat exchanger can be utilized in lieu of two of such heat exchangers in parallel as required in the prior art, by providing means to periodically bypass relatively cold, fully purified gaseous mixture normally flowing, through the crosscurrent heat exchanger to be cleaned, while maintaining the flow of the partially cleaned gaseous mixture therethrough to sublime and remove the congealed impurities.

It is therefore an object of this invention to provide a novel continuous process and apparatus utilizing a cross current heat exchanger for performing low temperature fractionation of gaseous mixtures in a more economical and compact manner than heretofore possible by providing means to clean the crosscurrent heat exchanger of impurities deposited therein in a fast and efficient manner, thereby obviating the necessity for two parallel countercurrent heat exchangers.

It is another object of this invention to provide a novel crosscurrent heat exchanger for low temperature separation of gaseous mixtures which is operable at a higher degree of efiiciency for a longer period of time than prior art heat exchangers by furnishing a novel distribution of heat exchange tubes throughout the cross-sectional area thereof.

It is still another object of this invention to provide a process for separating gaseous mixtures at low temperatures utilizing a crosscurrent heat exchanger wherein the heat exchanger is purged more quickly and less expensively than heretofore by furnishing means to periodically shut down the flow of refrigerant to the heat exchanger while maintaining the flow of warmer gaseous mixture to be cleaned therethrough to thereby vaporize and purge impurities deposited in the heat exchanger.

These and other objects of the invention will become more apparent to those skilled in the art by reference to the following detailed description when viewed in light of the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of one cycle of a separating system incorporating the features of this invention; and

FIG. 2 is cross-sectional plan view of a heat exchanger in accordance with the invention, in the scale approximately 1:2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, a gas inlet line 10 communicates with a turbocompressor 12 and, through a suitable switching arrangement, with pairs of reversible or regenerative heat exchangers 14a 14b, 14c and 14d. The latter are also called regenerators or heat accumulators. Gas line 10 is provided with means to switch communication thereof between reversible heat exchangers 14a and 140, as shown in the drawings, to the reversible heat exchangers 14b and 14d. Such switching and the apparatus for accomplishing the switching is well known to those skilled in the art.

The heat exchangers 14a and communicate with a crosscurrent heat exchanger, shown generally at 16, through switchable conduits 18 and 20 manifolded into a countercurrent heat exchanger inlet conduit 22. The aforedescribed switching apparatus is adapted to switch these conduits to the heat exchangers 14b and 14d concurrently with the switching of the gas line 10. Communication to the countercurrent heat exchanger 16 is controlled by a valve 24 in the conduit 22. The crosscurrent heat exchanger 16 is provided with an inlet chamber 26 which communicates, through a plurality of heat exchanger tubes 28, with a mixing chamber 30. Plates 32 and 34, disposed at the upper and lower ends of the tubes 28 respectively, enclose the area surrounding the tubes 28 and define therewith a zone 36 surrounding the tubes which communicates with the mixing chamber 30 through an orifice 38 in the lower plate 34. The mixing chamber preferably has such a volume that the speed of the gas leaving orifice 38 is reduced from approximately 0.4-2 m./sec. to 0.2-1 m./sec.

Referring again to the reversible heat exchangers 14c and 14d, conduits 40 and 42, controlled by valves 44 and 46 respectively, communicate with the heat exchangers intermediate the warm and cold ends thereof. These conduits are communicative with a transfer conduit 48 which,

in turn, communicates with the zone 36 in the crosscurrent heat exchanger 16.

A bypass conduit 50 communicates with the conduit 22 upstream of the valve 24 and, through a valve 52, temperature equalizing means 54 and turbine 56, with a fractionating column 58. The temperature equalizing means must have a large heat transfer area, preferably in the form of regenerator packing or adsorbents such as silica gel. The mixing chamber 30 of the crosscurrent heat exchanger 16 communicates with the conduit 50 through a discharge conduit 60, controlled by a valve 62, which, in turn, is connected to the conduit 50 downstream of the valve 52. A branch conduit 64 communicates with the conduit 50 upstream of the valve 52 and with a coil 66 in the fractionating column, and ultimately terminates through an expansion valve 68 in the upper portion of the column 58. Flow through the conduit 64 is controlled by a valve 70.

The lower portion of the fractionating column 58 communicates with the reversible heat exchanger 14!) through a product discharge conduit 72 while the upper portion of the fractionating column is similarly communicative with the reversible heat exchanger 14d through a discharge conduit 74. With respect to conduits 72 and 74, as well as conduits 18 and 20, there are provided conventional switching means for alternate pairing of the reversible heat exchangers to either the input or discharge side of the circuit.

An impurity discharge conduit 76 is communicative with the sump of the heat exchanger 16 and flow therethrough is controlled by a valve 78.

Turning now to FIG. 2 of the drawings, the crosscurrent heat exchanger 16 is shown in crosssectional detail in the scale approximately 1:2. This illustrates a preferred arrangement of the tubes wherein the tubes are spaced more closely at the periphery of the heat exchanger than at the center. By decreasing the tube spacing toward the periphery of the heat exchanger in such a manner, premature clogging of certain sections of the heat exchanger because of a non-uniform load upon the tube surfaces is prevented. In detail, the distribution of the tubes becomes apparent from the attached table. The free spaces between the tubes of the innermost circle is approximately 1.13 times greater than in the outermost circle. The values in the table and the dimension in FIG. 2 relate to a crosscurrent heat exchanger of 3.5 m. weight, having tubes with 30 mm. diameter therein.

In operation, with the reversible heat exchangers a and 140 precooled either by discharge from the apparatus or external sources, air, compressed in the compressor 12, is passed through the heat exchanger 14a and 140 for cooling and cleaning thereof. Fully cleaned and cooled air, exiting at a temperature, for example, from 85 to 110 K., is then directed through the conduits 18 and 20 and the conduit 22 to the inlet chamber 26 of the heat exchanger 16. The valve 44 in the conduit is maintained open in the switching cycle illustrated to withdraw partially cleaned, partially cooled air from the heat exchanger 14c intermediate the warmed and cooled ends thereof. As an example, 2 to 8% of the incoming gas is withdrawn from the heat exchanger 140 through the conduit 40 at a temperature of between 150 and 230 K.

The partially cooled air is transmitted through the conduit 48 to the zone 36 in the heat exchanger 16 for final cleaning thereof. It is found perferable to direct the partially cleaned air into the heat exchanger 16 through the zone 36 rather than through the tubes 28 since impurities frozen out of the gas being cleaned are more easily removed from the exterior of the tube surfaces than from the interior thereof. The partially cleaned gas then passes in indirect heat exchange relationship with the fully cleaned gas passing through valve 24 to congeal further impurities.

The almost completely cleaned gas from the zone 36 then exists through the orifice 38 into the mixing chamber 30, where it is mixed directly with warmed, fully cleaned gas which has passed through tubes 28. Because the loss of kinetic energy incurred by passing into the mixing chamber and since a lower temperature of the mixed gas is obtained as compared to the gas leaving through orifice 38, additional impurities are congealed from the gas to be cleaned, thus obtaining a higher degree of purity of the final product.

Preferably, only a portion of the fully cleaned gas from the heat exchangers 14a and 14c is diverted for cooling in the countercurrent heat exchanger. The remaining portion, preferably about 60 to is bypassed through the conduit 50 and mixed with gas leaving the heat exchanger 16 through the conduit 60. The mixture of gases from conduit 50 and conduit 60 is then fed into the temperature equalizing device 54 which preferably contains a regenerative mass of such character that carbon dioxide and hydrocarbons can be adsorbed therein. The mixed gas from the temperature equalizing device 54 then is expanded through the turbine 56 and discharged into the fractioning column 58.

A portion, preferably at least 50%, of the total input air of the gas flowing through the bypass conduit 50 is withdrawn through the conduit 64 to provide heat in the coil 66 for the sump of the fractioning column 58 prior to being passed through pressure reduction valve 68 and discharged as reflux liquid into the upper portion of the column.

Fractionation products withdrawn from the column 58 at the bottom thereof through the conduit 72, and from the head thereof through the conduit 74, are discharged through the reversible heat exchangers 14b and 14d for cooling same. When air is cleaned as in the described embodiment, the products of the fractioning column would typically be oxygen through the heat exchanger 14b and nitrogen through the heat exchanger 14d.

After the heat exchanger 16 has been operated for about one month, the load of congealed impurities, such, for example, as CO in the zone 36, is such that the heat exchanger must be cleaned. At that point, the flow of cold, purified air from the conduit 22 is interruputed by closing the vale 24; however, the warm, partially cleaned air from the conduit 48 is allowed to continue to enter the zone 36. Simultaneously the valve 78 in the conduit 76 is opened and the valve 62 in the conduit 60 is closed so that the warm air passing through the zone 36 vaporizes the deposited impurities and discharges through the conduit 76.

After a relatively short time, for example, approximately 10-12 hours, the tube surfaces of the heat exchanger 16 are free from congealed components, and the valves 24 and 62 can be reopened with the valve 78 simultaneously being closed to recommence the heat exchange operation in exchanger 16. As compared to total operating time, the cleaning time required by the process of this invention is so short that the temporary loss of 28% of the air to be fractionated does not unbalance the system to any significant extent The substances passing into the gaseous phase during the cleaning of the heat exchanger 16 may be discharged to the outside together with the partially cleaned gaseous mixture used to achieve the vaporization. This is particularly suitable when the congealed components, for example, CO are of little value, and do not provide an air pollution problem. Alternatively, the gas produced during the cleaning period of the heat exchanger 16 may be warmed, for example, in coils provided in the reversible heat exchangers 1411 through 14d, in order to make use of the cold inherent in this gas, and employed as instrument air, or for other purposes, as desired.

Although air is specifically referred to in the specifically described process and apparatus, it should be obvious that any gaseous mixture may be treated thereby.

The preceding examples can be repeated with similar success by substituting the generically and specifically TABLE Diameters of tube eircles (mm.) 275 350 425 575 650 725 800 875 950 Number of tubes 13 17 21 25 29 32 36 40 44 Free spaces between tubes (mm.) 36.4 34.6 33.5 32.8 33.2 32.8 33.2 32.4 32.1

Free cross-sectional areas within the tube circles (m 1.66 2.06 2.46 2.87 3.30 3.78 1.18 4.79 4. 5.38

What is claimed is:

1. An apparatus for low temperature separation of gaseous mixtures comprising:

a single crosscurrent heat exchanger having an elongated housing with a first inlet and outlet in opposite ends thereof, a plurality of spaced parallel heat exchange tubes disposed in said housing in concentric rings about the longitudinal centerline thereof and communicating with said first inlet and outlets to form a first heat exchange zone therein, means isolating the space between said tubes to form a second heat exchange zone therein, a second inlet in the periphery of said housing proximate said first inlet communicating with said second zone, a second outlet through said isolating means communicating said second zone with said first zone proximate said first outlet;

conduiting means including a source of low temperature .gas communicating with said first zone through said first inlet;

conduiting means including a source of relatively warmer impurity containing gas communicating simultaneously with said second Zone through said second inlet for cooling and congealing of the impurities therein by heat exchange with said low temperature gas; and means including a valve disposed between said source of low temperature gas and said first zone and a valve in said first outlet to interrupt the flow of low temperature gas to said first zone and to divert flow of gas from said second Zone to exhaust vaporized and purged impurities deposited in said second zone.

2. An apparatus in accordance with claim 1, further comprising at least one reversible heat exchanger for cooling and cleaning gas passed therethrough, said reversible heat exchanger having an inlet and outlet thereto, the outlet of said reversible heat exchanger comprising said source of low temperature gas, said source of warmer impurity containing gas comprising tap means disposed intermediate the outlet and inlet of said reversible heat exchanger to draw gas therefrom in a partially cleaned and partially cooled condition.

3. An apparatus in accordance with claim 2, further comprising a rectification column, a bypass means including a bypass conduit communicating a portion of the gas from said reversible heat exchanger directly to said rectification column, and means communicating said first outlet of said crosscurrent heat exchanger with said bypass conduit.

4. An apparatus in accordance with claim 3 wherein said bypass means further includes temperature equalization means and an expansion turbine communicative with said bypass conduit downstream of the point of communication thereof with said first outlet of said crosscurrent heat exchanger.

5. An apparatus in accordance with claim 1 wherein the first zone of said heat exchanger comprises an inlet chamber communicating with said plurality of heat exchange tubes and wherein the second zone comprises the zone surrounding said tubes, said second zone terminating together with the ends of said tubes in a mixing chamber, at least one outlet conduit means including a valve communicating with said mixing chamber.

6. An apparatus in accordance with claim 5 wherein the relative spacing between said tubes is increased toward the center thereof in such a manner that the free throughflow cross section outside the tubes increases slowly from the center to the periphery of the heat exchanger.

7. In a process for low temperature separation of gaseous mixtures wherein a gaseous mixture is cooled and cleaned from impurities contained therein in two separate parallel streams, the one stream being cooled to a lower temperature and being cleaned totally, the other stream being cooled to an intermediate temperature and being cleaned partially and wherein at least a first portion of said one stream and said other stream are brought into simultaneous indirect heat exchange with each other in a heat exchange zone in order to freeze out remaining impurities in said other stream the improvement comprising the steps of periodically interrupting the flow of the relatively cold gaseous mixture and continuing the fiow of the relatively warm, partially cleaned gaseous mixture to vaporize the frozen congealed impurities, and removing the vaporized impurities and the partially cleaned mixture from said heat exchange zone.

8. A process in accordance with claim 7 wherein the initial gaseous mixture is cooled and cleaned in at least one reversible heat exchanger prior to heat exchange of said one stream with said other stream and wherein said other stream is withdrawn from an intermediate point of at least one of said reversible heat exchangers.

9. A process in accordance with claim 7 wherein said one stream and said other stream after said indirect heat exchange therebetween, are mixed in a mixing zone to congeal additional impurities from said other stream said mixing zone being so dimensioned as to reduce the velocity and kinetic energy of said other stream as compared to the velocity and kinetic energy of same during said indirect heat exchange.

10. A process in accordance with claim 9 wherein said mixed streams are rectified after heat exchange therebetween.

11. A process in accordance with claim 10 wherein a. second portion of said first stream is diverted upstream of said heat exchange zone and mixed directly with said mixed streams prior to rectification thereof.

12. In a process for the low temperature separation of gaseous mixtures wherein a gaseous mixture is cooled and cleaned from impurities contained therein in two separate parallel streams, the one stream (22) being cooled to a low temperature and being cleaned totally, the other stream (48) being cooled to an intermediate temperature and being cleaned partially, and wherein said streams are brought into simultaneous indirect heat exchange relationship with each other in a heat exchange (16) in order to freeze out remaining impurities in said other stream (48) and the thus cleaned mixture is separated in a rectifier (58), the improvement comprising the steps of bypassing a major portion of said one stream (50) directly to the rectifier (58) so that the output thereof is substantially unaffected by periodic shutdown of the heat exchanger (16) for cleaning thereof, and during said shutdown bypassing remaining minor portion of said one stream (24) to said rectifier (58), employing said minor portion, during normal operation of said heat exchanger (16) to cool said other stream (48).

13. An apparatus in accordance with claim 1, further comprising at least one reversible heat exchanger for cooling and cleaning gas passed therethrough, said reversible heat exchanger having an inlet and outlet thereto, the outlet of said reversible heat exchanger comprising said source of low temperature gas, said source of warmer impurity containing gas comprising tap means disposed intermediate the outlet and inlet of said reversible heat exchanger to draw gas therefrom in a partially cleaned and partially cooled condition, and a rectification column, bypass means including a bypass conduit communicating a portion of the gas from the cold end of said reversible heat exchanger directly to said rectification column, and means communicating the outlet of said crosscurrent heat exchanger with said bypass conduit.

14. A heat exchanger for low temperature separation of gaseous mixtures comprising:

an elongated housing having at its frontal area an inlet communicating with a source of low temperature gas and an outlet thereto;

a plurality of heat exchange tubes disposed in said housing in longitudinal spaced relationship to one another and to the inlet and outlet of said housing and arranged in concentric rings about the longitudinal centerline thereof to form an inlet zone communicating through said tubes with a mixing zone proximate said outlet, means isolating the space between: said tubes to form a separation zone therearound, said separation zone having an inlet communicating simultaneously with a source of relatively warmer impurity containing gas through the periphery of said housing and an outlet communicating with said mixing zone at a point proximate the longitudinal centerline of said housing, means directing said relatively warmer gas normal to said heat exchange tubes.

References Cited UNITED STATES PATENTS De Baufre 62-13 De Baufre 62-13 XR Stoever 62-12 Rice et al 62-14 Rice 62-14 Rice 62-14 Linde 62-13 XR Newsome.

Potts 62-14 XR Matsch et al 62-14 XR Peters et al 165-110 Osborne 165-110 Cox 165-110 WILBUR L. BASCOMB, Primary Examiner US. Cl. X.R.

Patent No. 3,518,839 Dated July 7, 1970 Inventor s) RUDOLF BCKIR It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, Line 14, under "Number of Tubes" after "44" add ..4 Colu mn 5, Line 16, change "33.8,33.2, 32.8, 33.2"to

SIGNED'ANQ 5mm JR mmln, EMU-M col-union of Pat-Ont! Atteotingoffioor 

